This invention relates to an illuminator for illuminating multiple targets.
A known CMOS image sensor is composed of 16 million pixels uniformly distributed over an active area that is approximately 15 mm square. Each pixel has a sensitive region that converts incident radiation into electrical charge and is provided over its front surface with an optical element that includes a condensing lens for concentrating incident light on the sensitive region and a band pass filter for passing only a fairly narrow wavelength range, so that the pixel is sensitive to a fairly narrow range of wavelengths. In general, pixels of a first group are sensitive to red light, pixels of a second group are sensitive to green light and pixels of a third group are sensitive to blue light, and the pixels that are sensitive to a given color are uniformly distributed over the sensitive area of the device. When a pixel is illuminated with light of the wavelength to which it is sensitive, it generates a charge packet of magnitude that depends on the intensity with which it is illuminated. A readout circuit converts the charge packets generated by the pixels respectively to a voltage signal.
When a pixel is illuminated, its response depends on the sensitivity of the pixel and the intensity of the light incident on the pixel. Generally, it is desirable that all pixels of a given color should be of equal sensitivity, so that the response of each pixel of a given color to illumination of a given intensity is the same. Thus, if all the blue pixels, i.e. the pixels that are sensitive to blue light, are illuminated with blue light at a given intensity, the charge packets generated by the blue pixels should all be of the same magnitude.
It is desirable that a CMOS image sensor should be tested before it is incorporated in a larger product, such as a camera. A CMOS image sensor can be tested by illuminating the active area of the sensor with broad band light that is uniform in intensity and spectral content over the entire active area and has a high degree of flatness (the local uniformity or relative uniformity of illumination over adjacent pixels). Analysis of the voltage signal provided by the readout circuit then provides information regarding variations in sensitivity of the pixels of a given color.
Conventionally, a CMOS image sensor is tested using a point light source that emits broad band light. The point source is controlled so that its intensity does not vary substantially with time. The light emitted by the point source is filtered, integrated and collimated and then illuminates the active area of the image sensor. Using a point light source and optical conditioning, it is possible to maintain a high degree of field flatness and uniformity of illumination over the active area of an image sensor.
The cost of testing an image sensor is a significant part of the total cost of producing the sensor. One strategy for reducing the cost of image sensors, and thereby promoting demand for image sensors, is to reduce the cost of testing each sensor.
One conventional technique for reducing the cost of test of semiconductor devices having electrical inputs is multi-site testing. In multi-site testing, multiple devices are tested simultaneously in roughly the time taken to test a single device. In order to employ multi-site testing for image sensor testing, it is necessary that field flatness and uniformity be maintained over multiple sensors. However, it is difficult, or impossible, to maintain sufficient field flatness and uniformity of illumination over the active areas of multiple image sensors to allow multi-site testing of the image sensors using a point source and optical conditioning.
Use of an extended light source instead of a point source would allow a larger area to be illuminated, but an extended light source will generally vary in intensity over its extent and therefore the response of a pixel will depend not only on the sensitivity of a pixel but also on the location of the pixel.
In accordance with a first aspect of the invention there is provided a method of concurrently testing a plurality of image sensors, comprising (a) providing a light source emitting light, (b) supplying the light emitted by the light source to a first integrating sphere having an input port that receives the light emitted by the light source and having an output port providing output light, and (c) spatially dividing the output light of the first integrating sphere to provide a plurality of divided lights, (d) supplying the plurality of divided lights to a plurality of second integrating spheres respectively, each second integrating sphere having an input port that receives a divided light and an output port providing an output light, and (e) directing the output lights of the second integrating spheres onto active regions of respective image sensors.
In accordance with a second aspect of the invention there is provided apparatus for testing image sensors, comprising a first integrating sphere having an input port for receiving light emitted by a light source and also having an output port for emitting light, a plurality of second integrating spheres each having an input port for receiving light and also having an output port for emitting light towards the image sensors respectively, and a spatial multiplexer for receiving light emitted by the first integrating sphere and directing the received light to the input ports of the second integrating spheres respectively.